Effects of nutrient enrichment and small predator density on seagrass ecosystems: An experimental assessment

We used a held experiment to assess the individual and combined effects of removing top predators and enriching water column nutrients (nitrogen-N and phosphorus-P) on seagrass ecosystem structure and function. Experiments we re conducted in turtlegrass (Thalassia testudinum) habitats in St. Joseph B ay, FL, an aquatic preserve in the northern Gulf of Mexico that exhibits lo w ambient nutrient concentrations and contains abundant populations of smal l crustacean and gastropod mesograzers. We stocked 7.0 m(2) enclosures with elevated (similar to 4-8x ambient) densities of juvenile pinfish (Lagodon rhomboides), the dominant fish species in local seagrass habitats, to simul ate the first-order effects of large predator reductions, and we used an in situ delivery system to supplement N and P to similar to 3X ambient levels in nutrient addition treatments. Monthly determinations of water column nu trients and Chi a, along with measurements of the biomass and abundance of leaf epiphytes and seagrass production, biomass, and shoot and leaf densiti es were used to evaluate the relative effects of manipulating nutrient supp ly and altering food web structure. In contrast to our expectations, results showed few significant nutrient ef fects, or fish x nutrient enrichment effects on any of the parameters measu red. However, there were many significant fish effects, most of which were unexpected. As predicted, increased pinfish density reduced mesograzer numb ers significantly. Not anticipated, however, was the reduced epiphyte bioma ss in fish enclosure treatments, apparently brought about by the pinfish co nsuming significant amounts of epiphytes as well as mesograzers. This reduc tion in epiphyte biomass produced positive indirect effects on seagrass bio mass, shoot number, and rates of primary productivity in pinfish enclosure treatments. Our results also showed important top-down effects in determining the compo sition and abundance of seagrass-associated plants and animals in this pris tine environment. Although we did not observe simple trophic cascades, most likely because pinfish fed at more than one trophic level, and because the dense seagrass prevented small grazers from being reduced to low numbers, pinfish produced important changes in the epibiota as well as the seagrasse s themselves. These data, while contrasting with studies reporting signific ant negative nutrient enrichment effects on seagrasses, support the results of recent experimental studies in showing that: (1) small grazers can ofte n control the abundance of epiphytes; and (2) it is unlikely that a full un derstanding of the consequences of nun-lent enrichment for seagrass ecosyst ems can be gained without knowing how grazer population are regulated.